1. Field of the Invention
The present invention relates generally to improvements to optical fiber transmission lines, and more particularly to advantageous aspects of a dispersion-compensating fiber system having a bridge fiber and methods for making same.
2. Description of the Prior Art
As optical data transmission lines increase in length and in the amount of data that they carry, there is increasing interest in the development of new types of optical fiber and in techniques that can be used to refurbish already existing transmission lines. One important parameter of an optical transmission line is the amount of signal dispersion resulting from the optical characteristics of the materials used to construct the line. A new class of fibers has recently been developed known as dispersion-compensating fiber (DCF), which has a steeply sloped, negative dispersion characteristic.
One application for DCF fiber is to upgrade already existing fiber optic communication links. These already existing links are typically constructed using standard single-mode fibers (SMF) having dispersion characteristics that are optimized for operation at a signal wavelength of 1310 nm. However, certain applications require optimization of a communication link for operation at a longer wavelength, particularly where the communication link spans great distances. For example, one wavelength-division multiplexing (WDM) technique currently in use requires optimization of the link for operation at a wavelength of 1550 nm.
It is possible to refurbish an already existing SMF fiber transmission line optimized for operation at a given wavelength, such as 1310 nm, by splicing a length of DCF fiber into the transmission line. The length of the DCF fiber added to the SMF fiber transmission line is precisely calculated to produce an adjustment in the overall dispersion characteristics of the line such that it is now optimized for operation at a different desired wavelength, such as 1550 nm. A suitable technique for precisely calculating a length of DCF fiber to be spliced into an already existing line in order to optimize the line for operation at a different wavelength is disclosed in U.S. patent application Ser. No. 09/596,454, filed on Jun. 19, 2000, assigned to the assignee of the present application, the drawings and disclosure of which are hereby incorporated by reference in their entirety.
In addition to dispersion, a second important parameter for DCF fiber is the fiber""s loss value, that is, the amount of excess signal loss resulting from the introduction of the DCF fiber into a transmission link. Optimally, a DCF fiber should provide a highly negative dispersion, while only introducing a small excess loss to the fiber link. A useful index of the performance of a DCF fiber is the so-called xe2x80x9cfigure of meritxe2x80x9d (FOM), which is defined as the dispersion of the fiber divided by the attenuation.
Another important issue arising in connection with DCF fiber is the excess loss that results when DCF fiber is spliced to a standard single-mode fiber (SMF). To obtain a highly negative dispersion, DCF fiber uses a small core with a high refractive index, having a mode-field diameter of approximately 5.0 xcexcm at 1550 nm, compared with the approximately 10.5 xcexcm mode-field diameter of SMF fiber at 1550 mn. The difference in core diameters results in significant signal loss when a fusion splicing technique is used to connect DCF fiber to SMF fiber. It is possible to reduce the amount of signal loss by choosing splicing parameters that allow the core of the DCF fiber to diffuse, thereby causing the mode-field diameter of the DCF core to taper outwards, resulting in a funneling effect. However, the amount and duration of the heat required to produce the funneling effect result in an undesirable diffusion of dopant in the ring of refractive material surrounding the DCF fiber core. This diffusion of ring dopant limits the amount of splice loss reduction that can be obtained using a mode-field expansion technique. For example, using DCF fiber with a FOM of 200 ps/nm/dB, the splice loss typically cannot be reduced below 0.7-0.8 dB when splicing directly to SMF fiber.
There is thus a need for improved techniques for splicing DCF fiber to SMF fiber that reduces splice loss below current limits.
The above-described issues and others are addressed by the present invention, aspects of which provide an optical transmission line with reduced splice loss and methods for fabricating an optical transmission line with reduced splice loss. In a method according to the present invention, a length of dispersion-compensating fiber, or other suitable first transmission fiber, is spliced to a first end of a length of a bridge fiber. The splice is heated to a maximum temperature to cause a measurable reduction in splice loss. The temperature of the splice is then ramped down to room temperature, such that the reduction in splice loss is maintained. A second end of the bridge fiber is then spliced to a length of a second transmission fiber. A further aspect of the invention provides a technique for determining the maximum temperature for heating the splice between the dispersion-compensating fiber and the bridge fiber.
Additional features and advantages of the present invention will become apparent by reference to the following detailed description and accompanying drawings.